1. Field of the Invention
The present invention is generally related to charge injection and transport materials, and more particularly to hole or electron injection polymers and prepolymers, hole or electron transport polymers and prepolymers, method for forming the same, and their application in organic electronic materials and organic optoelectronic materials.
2. Description of the Prior Art
The working principle of organic light emitting diodes (OLEDs) or polymer light emitting diodes (PLEDs) is that the holes and the electrons which are injected from the anode and cathodes respectively. The holes will inject into the Highest Occupied Molecular Orbital (HOMO) of the hole transport layer, and electrons will inject into the Lowest Unoccupied Molecular Orbital (LUMO) of the electron transport layer. With the voltage difference across the OLED or PLED, the holes and electrons are able to migrate in the organic layers. Finally, electrons and holes recombine on the same polymer chain or molecule to form electron-hole pairs in the emitting layer, so that light emission can occur.
The most frequently used small hole transport molecules, such as N,N′-di-m-tolyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD) and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) have been proved to be excellent hole-transport materials and have shown a wide range of practical applications. These classes of materials offer many attractive properties such as high charge carrier mobility and ease of sublimation. However, they possess some disadvantages for use in long-lifetime OLED devices such as their relatively low glass transition temperature (Tg˜65° C. for TPD and Tg˜100° C. for NPB), ease of crystallization and unsatisfactory morphological stability.
Although polymeric charge transport materials can overcome the mentioned problems of small molecules, but fabrication of polymeric multi-layer structures is somewhat hard because typical solvents such as chloroform, dichloroethane, toluene or xylene would dissolve most electroluminescent (EL) polymers. Consequently, fabrication of multi-layer EL devices through layer-by-layer spin coating or inkjet printing would be difficult.
Therefore, new charge injection or transport materials are still needed corresponding to increasing thermal stability, decreasing injection barrier between charge transport layer and electrodes, providing easy fabrication method without dissolving the electroluminescent (EL) materials, and moreover, to improve the efficiency of organic electronic/optoelectronic devices.